This invention relates in general to an apparatus and method for spatially and temporally modulating a light beam and imaging the modulated light onto a photosensitive media.
Photographic images are traditionally printed onto photographic paper using conventional film based optical printer. The photographic industry, however, is converting to digital imaging. One step in the digital imaging process uses images obtained from digital cameras, or scanning film exposed in traditional photographic cameras to create digital image files that are then printed onto photographic paper.
The growth of the digital printing industry has led to multiple approaches to digital printing. One of the first methods of digital printing was the use of cathode ray tube (CRT) based printers. While such printers provide a means for digital printing, the technology has several limitations, for example reduced resolution as determined by the limitation of the phosphor and electron beam, high cost, and ability to provide sufficient red exposure to the media when operating at frame rates above 10,000 prints per hour. Another commonly used approach to digital printing is the laser engine shown in U.S. Pat. No. 4,728,965, which is a polygon, flying spot system using red, green, and blue lasers. Unfortunately, as with CRT printers, laser based systems tend to be expensive.
More contemporary approaches use a single spatial light modulator such as the Texas Instruments digital micromirror device (DMD), shown in U.S. Pat. No. 5,061,049, or a liquid crystal device (LCD) modulator to modulate an incoming optical beam. Spatial light modulators provide both significant advantages in cost, allow longer exposure times, and have been proposed for a variety of different printing systems from line printing systems such as the printer depicted in U.S. Pat. No. 5,521,748, to area printing systems such as the system described in U.S. Pat. No. 5,652,661. However, DMD technology is not widely available, is expensive, and is not easily scaleable to higher resolution.
Liquid crystal modulators are a low cost solution for applications involving spatial light modulators. Several photographic printers using commonly available LCD technology have been proposed. Examples of such systems are described in U.S. Pat. Nos. 5,652,661 and 5,701,185. Until recently, most spatial light modulators have been designed for use in transmission. While such a method offers several advantages in ease of optical design for printing, there are several drawbacks to the use of conventional transmissive LCD technology. Transmissive spatial light modulators generally have reduced aperture ratios and the use of thin film transistor (TFT) on glass technology does not promote the pixel to pixel uniformity desired in many printing applications. Furthermore, in order to provide a large number of pixels, many high resolution transmissive LCDs possess footprints of several inches. Such a large footprint can be unwielding when combined with a print lens. As a result, most LCD printers using transmissive technology are constrained to either low resolution or small print sizes. To print high resolution 8 inch by 10 inch images with at least 300 pixels per inch requires 2400 by 3000 pixels. Spatial light modulators with such resolutions are not readily available. Furthermore, each pixel must have a gray scale depth so as to be able to render a continuous tone print and do so uniformly over the frame size.
The recent advent of high resolution reflective LCDs with high contrast, greater than 100:1, described in U.S. Pat. Nos. 5,325,137 and 5,805,274, has opened possibilities for printing that were previously unavailable. Specifically, these references show a printer which is based on a reflective LCD spatial light modulator illuminated sequentially by red, green and blue, light emitting diodes (LEDs), and wherein the LCD spatial light modulator may be sub-apertured and dithered in two or three directions to increase the resolution. This method has been applied to transmissive LCD systems due to the already lower fill factor. Incorporating dithering into a reflective LCD printing system would allow high resolution printing while maintaining a small footprint. Also, because of the naturally high fill factor present in many reflective LCD technologies, the dithering can be omitted with no detriment to the continuity of the printed image. While devices such as the TI micromirror can incorporate a secondary mask as shown in U.S. Pat. No. 5,754,217, the mask may be displaced from the device or at the very least add to the processing complexity of an already complex device. The use of a single LCD serves to significantly reduce the cost of the printing system. Furthermore, the use of an area spatial light modulator sets the exposure times at sufficient length to avoid or significantly reduce reciprocity failure.
The progress in the reflective LCD device field made in response to needs of the projection display industry have provided opportunities in printing applications. One aspect of the design is that a LCD designed for projection display can be incorporated into the printing design with little or no modification to the LCD itself. By designing an exposure system and data path such that an existing projection display device requires little or no modification allows inexpensive incorporation of a commodity item into a print engine.
Of the reflective LCD technologies, the most suitable to this design, though not the only reflective LCD, is one which incorporates a small footprint with an integrated CMOS backplane. The compact size along with the uniformity of drive offered by such a device will translate into better image quality than other LCD technologies. There has been progress in the projection display industry towards incorporating a single reflective LCD, primarily because of the lower cost and weight of single device systems. See U.S. Pat. No. 5,743,612. Of the LCD technologies, the reflective LCD with the silicon backplane can best achieve the high speeds required for color sequential operation. While this increased speed may not be as essential to printing as it is for projection display, the higher speeds can be utilized to incorporate additional gray scale and uniformity correction to printing systems.
Current reflective LCD technology, however, does not provide adequate bit depth. Gray scale is a function of bit depth. Spatial light modulator printing systems can incorporate a variety of methods to achieve gray scale. Texas Instruments employs a time delayed integration (TDI) system that works well with line arrays as shown in U.S. Pat. Nos. 5,721,622 and 5,461,410. While this method can provide adequate gray levels at a reasonable speed, line printing TDI methods can result in registration problems and soft images. Alternate methods have been proposed, particularly around transmissive LCDs such as the design presented in U.S. Pat. No. 5,754,305, which can also be incorporated into reflective LCDs.
However, if the LCD is sufficiently fast, the proposed printer can create gray scale in area images adequately without time delayed integration or analog operation. It is the primary purpose of this invention to present such a solution.
An object of this invention is to provide for a high pixel density color image at an imaging plane. Another object of the present invention is to provide a high pixel density at a media exposure plane in an AgX printing systems. Yet another object of the present invention is to provide means to utilize a high site density spatial light modulator to create digital images for imaging onto photographic media.
Briefly, according to one aspect of the present invention, a method for improving exposure resolution that comprises several steps. A gray scale content of a high resolution image is analyzed and a determination is made of a media plane exposure resolution requirement. Exposure time and an exposure intensity is determined for each of the low resolution intermediate images. A number of low resolution intermediate images are required to provide the media plane exposure resolution; and an exposure time and exposure intensity for each of the low resolution intermediate images is determined. An image content for each of the low resolution intermediate images is determined and each of the low resolution intermediate images is printed at the exposure time and the exposure intensity.
According to one embodiment of the present invention light emitting diode sources are imaged color sequentially through a polarizer, spatial uniformizing optics, and a polarizing beamsplitter to create essentially telecentric illumination at the plane of a spatial light modulator.
A benefit of the present invention is that an 8 bit or lower resolution LCD, transmissive or reflective, used in a print engine, will result in acceptable contouring of the final output prints by providing an increase in the exposure resolution. This invention accomplishes this by using multiple exposures at exposure levels suitable for printing onto photographic paper.
The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below.